texts: Why were two galaxies named "The Sagittarius Galaxy"? Is this common?

texts:
Phys.org's  says:

  The formation of the Sun, the Solar System and the subsequent emergence of life on Earth may be a consequence of a collision between our galaxy, the Milky Way, and a smaller galaxy called Sagittarius, discovered in the 1990s to be orbiting our galactic home.

and I checked Wikipedia which says:

  Sagittarius Dwarf Irregular Galaxy Not to be confused with Sagittarius Dwarf Spheroidal Galaxy.

and

  Sagittarius Dwarf Spheroidal Galaxy Not to be confused with Sagittarius Dwarf Irregular Galaxy.

Question: But my question is how did they both get named after a very well known constellation? Is it common to name galaxies after constellations? Do many constellations have namesake galaxies?

  The Sagittarius dwarf galaxy has been orbiting the Milky Way for billions for years. As its orbit around the 10 000 more massive Milky Way gradually tightened, it started colliding with our galaxy's disc. The three known collisions between Sagittarius and the Milky Way have, according to a new study, triggered major star formation episodes, one of which may have given rise to the Solar System.  Credit: European Space Agency

Screen shot from the ESA video Dwarf galaxy collisions make stars form in Milky Way


texts:
Is there a standard mapping of symbols to terms for celestial and orbital mechanics (and physics in general)?

For example here are some ambiguities

Angular Momentum: L, H, J, $\Gamma$,

Orbital period: T or P,

M: mean anomaly or mass of the Sun,

E: energy or eccentric anomaly,

p: linear momentum or length of semi-latus rectum,

a: acceleration or length of semi-major axis,

e: eccentricity or base of natural logarithms,


texts: What would happen if Jupiter and Earth were at the same distance as the Moon is from Earth?

texts: If Spock came with a super machine capable of moving planets, and placed Earth so close to Jupiter as the Earth's moon, the gravity of Jupiter would make us fall into it?


texts: Question about number density of electrons

texts:
I somewhere see that in a white dwarf, we have this equation:

$\rho = 2 N_e * m_p / V = 2 n_e * m_p$

where $N_e$ is the number of particles (in this case e, electrons) and $n_e$ is the number density. and $m_p$ mass is mass of proton. My problem is with the 2 in this equation. why $2 * n_e$ and not just $n_e$?

Also about Electric Potential Energy between a Ion and electron, somewhere said:

$ E = (Z e * 1e) * n_e^{1/3} / (4 \pi \epsilon_0) $

I don't understand the $n_e$ term. Is the above really a equation or is it an approximation? I think it can't be an equation  because $ne \neq 1/r^3$.


texts: How to convert sidereal time to local time?

texts:
I came across something that won't let me go, but no matter how much I calculate (which I do not like), it doesn't seem to work.
Also: I've used so much apps and links already to convert, but it doesn't work in that direction. You see, local time converting to sidereal time = not that hard.

I need to know the local 'normal' time of my local sidereal time.
The sidereal time is exactly 13.30h, from that I would like to know the normal local time.

The time zone = Brussels (CET)
Olson time zone Europe/Brussels
Time difference: same as Amsterdam and Europe UTC+1
no summer time at the moment (and what if that was the case, what would the local time be then?)
Latitude: 50,929315°
Longitude: 5,337367°
Address: Grote Markt 18, 3500 Hasselt Belgium.

The reason I need to know this has to do with a sort of awareness I found in ancient writings and books. It has extra power than to meditate, and I would like very much to try this out.

But of course, I need to know the local time then.


texts: How to calculate the S/N of IFU or long-slit galaxy observations

texts: I would like to calculate the Signal-to-Noise ratio (SNR) of long-slit or integral field unit (IFU) observations of H$\alpha$ emission. I can calculate the SNR or each individual spectrum, just fine. However, how can I assign a single SNR value to an observation? Do I just quote the higher SNR or do I calculate the median/mean SNR of all spectra? Is there a convention for this? My data is low-redshift galaxy IFU observations, and thus the SNR is higher in the center of the galaxy and lower at larger radii.


texts: How moon is visible during sun is still shining, simple explanation please!

texts:
Sometimes I observed there is moon on sky at noon and sun is also shining.

How is this possible?

I also observed dull moon is still on sky at 7:00 AM to 8:00 AM while sun is rising. My local time is UTC+5.

If possible, please explain it in simple words without using advanced astronomy terminology if possible, thanks!


texts: When analyzing an image I'm used to thinking of the pixels as a grid of boxes the photons have fallen into. I assume that there's zero space between the boxes. That seems obviously wrong, since there needs to be some separation on the detector to prevent electron bleed between pixels. So, what's the real geometry of pixels in a detector like (especially for: CCDs and infrared arrays)? Is it closer to a grid of points than a grid of boxes?


texts: What would happen to the Sun's evolution if it collided with another yellow dwarf?

texts: How would the Sun evolve if it collided with another G-type main sequence star, perhaps a more massive one like Alpha Cen A? Since its mass would then be above the Chandrasekhar limit, could it eventually become a neutron star instead of a white dwarf? Or would it lose enough mass until then so that it would still become a white dwarf, just like Sirius A (whose mass is above the Chandrasekhar limit too but will develop into a white dwarf anyway due to mass loss)? Would the Sun's yellow-dwarf-to-red-giant phase be prolonged due to the additional mass? When largest, would the red giant be bigger than the currently estimated 256 solar radii?


texts: I live around Houston, Texas and I'm taking advantage of Earth hour to look at the stars. I've found two objects that I think might be Mars. Both of them are in roughly the same plane as Venus and Jupiter. Both are closer to Venus than Jupiter. One of them has a reddish tint and is higher in the sky than Venus, and the other is closer to the horizon than venus. It is 8:50 as I type this. Is one of these Mars?


texts: Do all total solar eclipses have the ring of fire when observed from ground?

texts: I was recording the phenomenon and then I soon realised that the ring of fire don't dissappear, is it because of the apparent size of the moon and sun as well as the relative distance so that every eclipses I see would alway have the ring of fire on it? Also I ruled out lensing because the moon is not massive enough right?


texts: Is there a known relation to patterns of star formation in a molecular cloud?

texts:
I was wondering if there was a known pattern to what mass of stars can form in a molecular cloud depending on what had formed before them?
Having read that when blue giants form and go supernova in a relatively short period compared to other stars main sequence time scales and when these massive blue giant go supernova the force of their explosion can create new star formation activity as the supernova pushes on the molecular clouds gas structure. This made me wonder if there was a known chain reaction of specific masses of stars that will form or even without the blue giants forming if there was a known pattern of what stars will form given enough density in the molecular cloud and if the chain reaction of forming stars has a known pattern?


texts:
I have a Skywatcher 130/900 and I found that if I do an accurate polar alignment I'll need only one motor to track a celestial body, so I decided to build my motor. Right now I'm facing a problem with the mating of the spur gear that drive the worm gear

Using a caliper I found that the gear has the outer diameter of 50mm, and a tooth is 1mm in height, so the reference diameter is 49mm; the gear has 140 teeth, and so the module (reference_diameter / n_of_teeth) is 0.35.
After this I used Solidworks to model a spur gear that could match it, and printed this

When I tried to check the mating, I found this

The teeth seems right, but they're too short and can't mate.
I used a caliper and counted the teeth to obtain the gear specs, and I think is kinda stupid as approach, but I could not think of something better to mate my gear. Is there a better approach to this?
Then, the idea was to try and remove the gear (removing the screw) and replace it with a 3d printed gear, but it does not seems to work (and I think it could break the telescope mount). Did someone try to do it?
Searching online I found that (obviously) there are some tracking motors and another question came to my mind: how can they mate the gear? Is there some kind of standard? Does it mean that every telescope's mount has some kind of standard gear? If yes, where can I found them?
EDIT: That's the situation: on the right there is the spur gear that I'm trying to mate (or eventually remove, if possible) and in the center there is the worm gear that move the telescope on the RA

EDIT2: Here's the project of an involute gear with Solidworks. (I followed this tutorial) but as you can see, the teeth overlap.

As a reminder, I have the following parameters

Pitch diameter (PD): 49mm (caliper measurement)
Number of teeth (Z): 140
Module (PD/Z): 0.35

Am I doing something wrong (or am I missing something)?


texts: Do orbital resonances always form naturally?

texts: For example, if I throw two planets to orbit a star at random direction, would they form an orbital resonance?


texts: How would the characteristics of a habitable planet change with stars of different spectral types?

texts:
The tidal forces a habitable planet experiences increase with decreasing spectral type. So a habitable planet orbiting a smaller, less massive, and cooler star would experience much stronger tidal forces than one orbiting a bigger, more massive, and hotter star, and likely be tidally locked.
But what about other factors? How do other characteristics scale up with different stars?
For example would the orbital velocity of a planet in the habitable zone of a red dwarf be smaller than the velocity of a planet in the habitable zone of a more massive and hotter star?


texts: How has the Earth's orbit changed over hundreds of millions or billion of years?

texts:
First, I know that modeling orbital mechanics of 8 planets is hard, but there are some theories out there, for example, Jupiter is thought to have moved in towards the sun then started moving away.   Article

and Uranus and Neptune may have switched spots Article

Is there any pretty good evidence on how the Earth's orbit has changed over time.   I remember reading some geological evidence that a year used to be longer, implying that the Earth used to be farther form the Sun, but I've since been unable to find that article, and for purposes of this question, lets count a day as 24 hours even though a day used to be quite a bit shorter hundreds of millions or billions of years ago.   - footnote, I've still not been able to find that article but it occurs to me, it could have been counting shorter days, not longer years - so, take that part with a grain of salt.

Are there any good studies out there on how many 24 hour days were in a year, 100, 300, 500, 800 million years ago?  or 1 or 2 billion years ago?  Either geological or orbital modeling?   Preferably something a layman can read, not something written by and for PHDs?

Or any good summaries, also encouraged.    Thanks.

I also found this article, but it seems more theoretical than evidence based.   http://www.futurity.org/did-orbit-mishap-save-earth-from-freezing/


texts:
I was fortunate enough to see the solar eclipse in southern Illinois last year. The clouds were not friendly, but we did have one spectacular moment of perfect eclipse. It is an eerie sensation, and I cannot imagine how utterly terrifying it must have been for our ancient ancestors to see the moon eating the Sun.

However, it got me wondering. A solar eclipse can occur in the spectacular fashion it does because the angular dimension on the moon and the Sun are almost exactly the same -- this despite the fact that they are dramatically different in actual dimension. This seems a remarkable coincidence to me -- the ratio of the distances being almost exactly the same as the ratio of their diameters.

I was wondering if there are any other known examples of such a phenomenon occurring anywhere else in the universe? Can we make any estimate of how often something like that might occur in a galaxy?


texts: What happens if you fail to reach escape velocity for the galaxy?

texts:
I was listening to Skeptics Guide to the Universe the other day, and Steve Novella mentioned that the escape velocity for the galaxy is 544 km/s.

If an object fails to reach escape velocity on Earth, it will return to Earth.  What will be the fate of an object, say the Voyager spacecraft which is currently traveling at 17 km/s, that fail to meet the 544 km/s.

I'm having a hard time wrapping my head around what galactic escape velocity means.

Does this mean that Voyager is incrementally slowing down by the pull of the galaxy?  Given enough time, will it end up in the supermassive black hole in the center?


texts: Can a human in a normal astronaut suit designed for the moon, land on Mercury and walk on it's surface. If not what would happen to him/her?


texts: very bright object (visible even after sun risen)

texts: Brevard Co. FL. At first I thought it was a comet it appeared to have a tail. Starts out low in the east and gains elevation as the sun rises. As the sun is just above the horizon it is still visible at about an 80 degree elevation. What am I seeing?


texts: How can the author of this article conclude that earth-shine on the moon is 100 times brighter than moon-shine on earth?

texts: I don't understand the reasoning of the author of this article, on page 26. How can he conclude that full earth-shine on the moon is 100 times brighter than full moon-shine on earth?


texts: Is cosmic shear generally agreed to have been observed?

texts:
A review by Martin Kilbinger, 2015 says:

  Cosmic shear is the distortion of images of distant galaxies due to weak gravitational lensing by the large-scale structure in the Universe. Such images are coherently deformed by the tidal field of matter inhomogeneities along the line of sight.

As a rough analogy: imagine looking at an image full of galaxies through a rippled piece of glass. While a circular lens might make a galaxy look smaller or larger, a ridge would have a preferential direction of magnification, and the galaxies under that ridge would have a statistical tendency to have slightly different sizes on average parallel vs perpendicular to the ridge.

Is such a phenomenon generally agreed to have been observed yet? Are there any plots (of any kind) that show such experimental data?

If I take this rough analogy one step further, while the peaks and troughs of the ripple magnify and demagnify, the midpoints between would be more prismatic - they would tend to deflect the centers, or locations of objects systematically, rather than affect the shapes. Applying this to cosmic shear, wouldn't the existence of shear essentially suggest that - to put it simply - stuff is, in fact, "not quite" where it appears to be?

above: from Free Ripple and Frosted Glass Overlays, Bojan Živković.


texts: What is a "fundamental location", and why is Mosting A defined as one for the moon?

texts: The crater Mosting A was chosen to be the "fundamental location" for the Moon's coordinate system (latitude and longitude). But what exactly is a "fundamental location"? What does it have to do with the establishment of a coordinate system? Is it just so that observers have a way of easily projecting a spherical grid on the moon? And why was Mosting A chosen for the job?


texts: Exoplanet hunters ignored or ridiculed in the 1980s or 1990s?

texts:
I recently came across this youtube video, in which a few people are singing about the history of exoplanet discovery.

It suggests that, in the 1990s, exoplanet hunters were largely ignored or ridiculed. Is there any truth to this?


texts: Since Mars is only weakly magnetized does Mars not suffer from  ground induced currents or are they far worse than on Earth?


texts: What is a "jumping trojan"? And what do their orbits look like?

texts:
I was googling Neptune's trojan companions and found on Wiki this claim about  ~200 km diameter (316179) 2010 EN65:

  " the object is actually a jumping trojan, is jumping from the
  Lagrangian point L4 into L5 via L3."

So some asteroids not only oscillate between L4 and L5, between being trojans or greeks, but even go via L3, on the opposite side of the Sun? Are their orbits circular as Neptune's (in this case) or highly eccentrical? How could an object in the same orbit as Neptune have another orbital period than Neptune? Or how else can it migrate between L3, L4 and L5?

And are "trojan" objects expected to gather in L3 too?

EDIT:
I'd like to add these illustrations because I think they illustrate well how a regularly orbiting object seems to "stay" longer at L3, L4, L5. (I'm not sure whether "jumping trojans" have some significant other characteristics, though).


texts: Mass required for a certain fusion process

texts:
Suppose we have a density model
$$ρ = ρ_c (1-r/R)$$
where $ρ$ is density, ${}_c$ means central, and $R$ is the stellar radius.
Let's call the temperature for that fusion process to begin, $T_f$.
I want to know the minimal stellar mass for a certain fusion to begin.
I made some assumptions, for example the values of X, Y, and μ. So if the gas is non degenerative and non relativistic, we have
$$P = \dfrac{ρκΤ}{μm_h}$$
But im not sure what to do with the pressure. I have $T,μ$, can obtain $ρ(M,R)$. Maybe I can use virial?


texts: If black holes swallow stuff, will a big black hole eventually swallow the universe? The universe will eventually go dark, because there will not be enough matter to make stars, and all that will be left is a bunch of black holes and they can merge. So will the universe get swallowed by a big black hole someday?


texts: Should BD in this sector means Brown dwarfs or Black dwarfs?

texts:
The one in the sector as BD/NS/BH. And please explain why Brown/Black dwarf should be here among neutron stars and black holes. Thank you for your help.

Source: Beech, M. (2019). Introducing the stars. Cham: Springer Nature Switzerland AG.


texts: Orbital plane evolution and planet discovery via transit method

texts:
I'm aware that our solar system is largely/roughly on a single orbital plane, but haven't other star system's orbital planes been scrambled by billions of years of interaction?

If so - how much chance is there that an exoplanetary system's orbital plane is inclined favourably enough to witness the dimming of a transiting planet across its sun?


texts: Can we calculate the average temperature of the heliosphere?

texts:
It is my understanding that one can measure the temperature of the universe by measuring cosmic microwave background radiation. Could we also use this method for calculating the average temperature of the heliosphere?

The average temperature of the universe has supposedly been measured in at 2.735 degrees above absolute zero. Presumably this measurement takes into account the massive amounts of 'empty' space which I imagine has a huge effect on the number.

My question: Can we measure the average temperature of the heliosphere? If so, then what is it?

This might be quite hard to define as how do you determine when our solar system ends? So for the purpose of this question the following assumptions should be made:

The distance from the Sun to the edge of the heliosphere is approximately 100AU (Roughly the size of the heliosphere as measured by Voyager 1).
Assume the heliosphere is a perfect sphere.
Assume we are measuring the black body equivalent temperature (as this is what was measured to calculate the temperature of the universe).

If we don't know how to measure this average temperature (which is likely if we can't differentiate between microwaves from in and outside the heliosphere) then how could we theoretically measure this value assuming a perfect environment?


texts: If an object 1 billion light years away emits light, does it take more than 1 billion years to reach us because of the expansion of the universe?

texts:
From page 7 of the recent (September 26, 2020) edition of Science News Magazine:

Detected May 21, 2019, the gravitational waves came from a source about 17 billion light-years from Earth, making this the most distant
confirmed detection.  Because of the universe's expansion, that
distance corresponds to a travel time of about 7 billion years,
meaning the gravitational waves were emitted when the universe was
about half its current age.

But that means the object(s) were only about 4 billion light-years or so away when the merger of two black holes occurred, right?  But it take 7 billion light-years, rather than approximately four, for the gravitational waves to reach us because of the expansion of the universe itself?
Is there an online calculator that can calculate this?  Does anybody know the equation(s) interconverting current distance, original distance and travel time of the radiation?


texts: When will solar eclipses become lunar transits?

texts: When will the moon be far enough away from the Earth that we would no longer consider it a 'solar eclipse', but rather a 'lunar transit' when the moon travels between the Earth and Sun?


texts: Right ascension, Equinox, Positions Help

texts:
I am trying to answer the following question which I slightly understand but need a clearer explanation:
What are the times of year to observe an object at X hours of RA at visual wavelength?
I am trying to use the march equinox as a starting point for 0 RA and for a 12 hour RA object, it will begin to rise from the east as the sun sets west of course, around 6 months from the march equinox???
I appreciate an explanation and if you can link a map or a helpful visual as well. Thanks!


texts: What exactly is a fortney grid transmission spectra modeling

texts:
I am trying to model some exoplanets and I don't understand how this model works. Below is a link of the model. What I don’t understand is how does graphing the transit depth  vs wavelength provide information. I am trying to determine the atmospheric chemistry/atmosphere/properties of planets.
https://exoctk.stsci.edu/fortney


texts: Are stellar spectra unique enough that they could be used to identify particular stars? If so, has any attempt been made to use them for navigation or equipment alignment?


texts: what does it mean of different eyepieces?

texts:
what is the difference between 4mm and 24mm and 30mm eyepieces?
could I buy an eyepiece from another brand?
I have the Gskyer telescope.could I buy for Celestron eyepieces?

thanks.


texts: Moons of migrated giant planets: Icy, Rocky, or none at all?

texts:
My understanding is that gas and ice giants form outside the "snow line" of their protoplanetary disk, then sometimes through gravitational interactions they are able to migrate into the inner star system.

In this situation, if the giant planet had moons, what would they be like?

Would the planet be able to bring initial moons with it? Or would they be lost?
If they are kept, are they likely to retain an icy composition due to forming outside the snowline?
If not, can it form or capture new moons post-migration?


texts: Will just a glimpse (less than a second) of sun during partial solar eclipse damage eyes?

texts:
This doubt arose when I thought that on Normal Days, at least I have observed bright sun while going outside, going to office, in park etc. for less than a second. It happens almost daily. Not deliberately but that's how things work I guess. Similar example is playing cricket. Many times sun comes in the way of ball in the air and again I have a glimpse of sun. To me, it is very common as I've seen in international cricket matches on TV.
On the other hand, in case of the days of Solar Eclipse, on one or two occasions, when the sun wasn't very bright due to clouds, I've deliberately observed the partial eclipse just for a glimpse, just to confirm it's actually cut. And I confirmed it. (How fast it was? It was not like I stood still and observed the sun for less than a second and then closed eyes. It was like this: I started moving my head and sight from earth surface and in a faster way moved my head towards sun and without any break, stopped at safe location on earth again where there was no sun). And it did literally no harm to me.
So my doubt is:
Why there's so much theories that you must not stare sun even for less than a second or even a glimpse by mistake? If you see news on eclipse day, you'll see full news about this. But on any other normal day, no such news. Is there something special, something more bright on solar eclipse day? Will it damage eyes?


texts:
Earth receives shower of both solar and cosmic radiations every seconds. The Sun put up a heliosphere around the solar system and thus foreign high energy charged particles could not penetrate easily. Is there any other source most probably that lies within our solar system?

Note:

Very high energy cosmic particle beyond 5.7 x 10^19 eV were observed
by Pierre Auger Observatory in Argentina. (rare but the GZK limit
seems to rule out distance sources)
GZK limit roughly capped the upper energy level of cosmic ray as
they are slowed due to CMB over vast distance


texts: What are the current observational constraints on the existence of Nemesis?

texts: Nemesis is a hypothetical companion to the Sun on a very eccentric, long-period orbit. The star supposedly returns every few tens of millions years, driving comets into the inner solar system and causing extinction events. Given our very stringent observational limits from infrared surveys (such as WISE), is its existence definitively ruled out?


texts: Are galaxies moving away from us faster than before?

texts: Are all galaxies moving away from us at constant speed even those that may be moving in our direction as the space is being formed? How does nothingness appear to push matter? A black hole sucks as space is compressing (being stretched without stopping) or compressed (like how our Sun pulling all planets) and is it the opposite phenomenon that show universe is expanding?


texts: Plot an AltAz grid over a square grid of RADec points

texts:
first post here.  As I'm new, StackExchange won't let create or use the wcsaxes tag.  wcsaxes looks like the most appropriate tool for the job, but astropy is closely related.

I think the title says it all, but I'll give a little more detail.  I have a bunch of sources in (RA, Dec) and want to plot them in the simplest possible projection (i.e. square, but if this is not possible we can make allowances).  I want to see the geometry of the Earth over my region of interest, mostly to identify the Earth's magnetic field lines.

The following code gets me close, but I get this error:

AttributeError: 'NoneType' object has no attribute 'to_geodetic'

If I change "altaz" to "galactic", I get a Galactic coordinates grid over the points, which is what I want, but obviously in the wrong coordinate frame.

#!/usr/bin/env python2

import numpy as np
from astropy.wcs import WCS
from astropy.time import Time
import matplotlib.pyplot as plt

# time = Time(2606629, format="jd", location=("116.670810d", "-26.756528d")).iso
w = WCS(naxis=2)
w.wcs.ctype = ["RA---MER", "DEC--MER"]
# w.wcs.dateavg = time

ra_min = 0
ra_max = 15
dec_min = -45
dec_max = -15
N = 1000

sim_ra  = np.random.uniform(ra_min, ra_max, size=N)
sim_dec = np.random.uniform(dec_min, dec_max, size=N)

fig = plt.figure()
ax = fig.add_axes([0.1, 0.1, 0.9, 0.9], projection=w)
overlay = ax.get_coords_overlay('altaz')

overlay[0].set_ticks(color='white')
overlay[1].set_ticks(color='white')
overlay[0].set_axislabel('Longitude')
overlay[1].set_axislabel('Latitude')
overlay.grid(color='black', linestyle='solid', alpha=0.5)

plt.scatter(sim_ra, sim_dec)
plt.xlabel('RA')
plt.ylabel('Dec')
plt.show()

I played a little with trying to get the observation time into the WCS header (note that the actual time is artificial, but should work regardless), without success.  Any ideas?


texts:
According to the orbital plane XYZ, I understand that the axial tilt is the angle between the planet axis rotation and any normal of the plane (actually Z in right handed system). BUT, this is not sufficient to define a vector direction, I need another angle into the XY plane. Where do I find this information?  Actually I believe it's relative to the direction of the periapsis. Am I right?

Following all the nice conversation on the answers, I maybe need to ask

How to get the axial tilt vector(x,y,z) relative to ecliptic or orbital plane, given ONLY ONE angle, i.e. 23.439281 deg for the Earth, 25.19 for Mars, etc.


texts: I have uploaded a h alpha photo in SAO Image DS9, but want to locate a particular region for which I have the galactic coordinates. How to input the coordinates in DS9 to view the particular zone?


texts: Difficulty in using the telescope as a projector

texts:
I have been trying, but completely failing, to copy the arrangement for viewing the sun mentioned on this website:  https://astronomyconnect.com/forums/articles/2-three-ways-to-safely-observe-the-sun.21/

The telescope shown does not appear to have a finderscope or any other obvious means of finding/tracking the sun.  This set up is actually far harder to do than it would appear in the photo.  However, I think it is worth the effort as it doesn't risk blinding the observer.
Finding the sun is not our issue because our telescope moves.  We punch in what we want to look at and the telescope moves there.   We can use it manually, but have found that the sun is surprisingly difficult to find without looking through the finderscope.   We are not willing to do that for obvious reasons.
I have been trying to figure out a way to have the board portion grab hold of our telescope instead of set on a table.  So when the telescope moves, the board moves with it.  Orion does sell a sun-filter, which I have purchased, but I am not willing to set a precedent with my homeschoolers where they look at the sun through the telescope - even with a sun filter.
Does anyone know of another website with instructions to use the telescope as a projector that uses a telescope similar to mine?


texts: Code/Package for multiple planet simulation

texts: I want to do a quick study and some tests on the planet-star dynamics ( including the movement of the star). I'm looking for a code that simulates some planets orbiting a star. I've tried searching online and the ones that I found are either incomplete or do not seem to give realistic results. With that in mind I'm wondering if there is any code/package that does this already (in Python).


texts: How to calculate emission measure from EVE data?

texts:
I am trying to analyse solar flare temperatures from EVE FITS files in PYTHON. All I have are the irradiance values for a range of wavelength at every 10 seconds.
I have made clear Gaussian fits to the line profiles but not able to figure out how to calculate the emission measure (EM). All I know is that it is the integration of plasma volume times the square of electron density. I have Gft function files that gives the Gft values at density $10^{11}$ but not sure what to do with those (except make ratio temperature plots to estimate temperatures in impulsive and gradual flare phase).
Any suggestions? I have tried reading numerous papers but nothing helps.


texts: Are there any moons of minor planets that orbit at high inclination to the plane of rotation of the parent body?

texts:
It occurred to me that one of the ringed asteroids or minor planets may answer this question about the alignment of planetary rings. However, it appears that the rings of Chariklo, Haumea and Chiron (possible rings) are within a few degrees of the plane of rotation of the asteroid.
My thinking was that minor planets with low gravity may be able to capture/maintain moons at a higher orbital inclination than larger planets, and those moons will not be in distant orbits as described here.  Satellites could also be formed by collisions and their spin evolution is described in this article.  I think there may be an argument that the YORP effect and other tumbling or precession effects could lead to unusual orbital attitudes for natural satellites perhaps?
There are four hundred and sixteen minor planets that have moons, are any of those moons known to orbit at high inclinations to the plane of rotation of the parent body?


texts: How big is one arcsecond at various distances?

texts:
I've been reading this article
https://en.wikipedia.org/wiki/Celestial_coordinate_system
and a question came to my mind.

The minimum units are seconds ( ″ ). I picture an angle of 1 second as being like a "cone" going out from Earth. It's really small at the beginning but as you go further its width increases.

How wide does this cone get as you get further into the universe? How big is an arcsecond on different astronomical bodies?


texts: What does it mean for a star to go nova or supernova? Can I safely observe these?

texts:
What does it mean for a star when people say it goes 'nova' or super nova, what are the differences?

More importantly, can I safely observe these with an amateur telescope? I imagine they would be big enough to spot should one happen!


texts:
I am not talking about an antigravity effect that would cancel gravity, rather if there is a repulsive force, like when you use two magnets and flip one of them, where it becomes obvious that one is pushing away the other.

Hope the question is a good fit for the site, feel free to migrate it otherwise.


texts: Could a magnetars magnetic field have saved Mars atmosphere?

texts:
Just wondering: if a magnetar was close enough to Mars would it have saved its atmosphere by protecting it from solar radiation? I was wondering this because Earth protects us via a magnetic field; the magnetar would be providing Mars one. This is if it did not destroy the planet first as suggested here.

Edit
I meant if the magnetar was safe to live by.


texts: What caused this mysterious stellar occultation on July 10, 2017 from something ~100 km away from 486958 Arrokoth?

texts:
In 2017 a series of stellar occultations by asteroid 2014 MU69 "Ultima Thule" now officially named 486958 Arrokoth were timed in order to obtain better orbital information before the New Horizons flyby and to look for additional large chunks of debris that might be of scientific interest and pose a danger of collision thereby warranting a more distant flyby.
There was a pleasant surprise in the shape of the asteroid determined from an array of portable telescopes deployed across the occultation paths. The asteroid appeared to be binary, which was puzzling because there were no obvious oscillations in its light curve, but that's a different story.
Below is an assemblage of three such occultation measurements. We can see the asteroid's double circle shape nicely. But what interests me here is the single trace for July 10th. It seems to show an occulting object clearly distinct from yet incredibly close to Arrokoth. A pure coincidence would be more than extremely unlikely as the time window is seconds and the distance less than 100 km at ~45 AU!
Question: But I don't remember reading about a separate secondary body orbiting Arrokoth. So what caused this apparent occultation on July 10, 2017?

From this answer to Will the upcoming observations of occultation by Arrokoth (2014 MU69) be of a single object, or two?


texts: Convert Stellarium video capture to 360 for youtube

texts:
I am wanting to capture video of Stellarium (I can already do this with CamStodio) and somehow convert this into a video usable for the YouTube 360 (or VR!) capability.

Even if it can be converted into 180 and I need to add an image of the ground or spaceship to fill the rest of the image, that would still be super helpful!

I have found this, which gives a good overview of the maths of converting various projections (I haven't gone through it in detail yet), but I was wondering if someone has already written something to do this, before I go and write my own code for it?

I already use Davinci Resolve for my video editing, and I'm on a Windows 10 Pro (with access to various Linux machines).

Cheers!


texts: How to search binaries in Simbad using spectral types

texts:
In the Simbad database there are many binary stars listed with the spectral classification of each component. For example, for $\alpha$ Centauri, Simbad yields a spectral type of G2V+K1V.

My question is, what criterias do I have to input to search for all the binaries with a G and K component? And what If I want to search specifically for GV + KV binaries?

I don't know if this search can be done with the sptypes and splum search criteria.


texts: Theoretical measure of distance to black hole?

texts: Would it be possible to measure the distance to a black hole of known mass by measuring the radius of the most redshifted radiation that is detectable? The idea is that since the point where that radiation vanishes is related to the gravitational potential difference between the observer and the point of emission of that radiation, we should be able to get an estimate of how far we are by observing how far into the black hole we can see (assuming that for a given mass and an observer at infinity that radius is fixed).


texts: Is the Universe really expanding at an increasing rate?

texts:
Here's what I just read from Wikipedia's page on the Hubble Space Telescope:

  While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project used ground-based telescopes and HST to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. The cause of this acceleration remains poorly understood; the most common cause attributed is dark energy.

I don't take this at face value because we should expect more distant objects to have higher observed speeds and therefore higher observed red-shifts. Here's why.

Let's start with a model where the Universe expanded very fast early on, but has been slowing down ever since due to gravity, as one would normally expect.
Remember that, the farther away a cosmic object is, the farther back in the past we are observing it. An object 1,000 light years away, if it's light is reaching us now, is being observed in its state that existed 1,000 years ago. We are effectively looking through a time machine.
So if we observe a more distant object, we're observing an older state of that object. Therefore, we are observing it at a time when the Universe was expanding faster than it is now, so it has higher red-shifts.

And isn't that what we observe today? The more distant the galaxy, the higher its red-shift? This is not inconsistent with a "normal" model where the expansion is slowing down due to gravity.

What am I missing? Why are scientists trying to explain such things with weird dark matter and dark energy that otherwise have never been detected or found evidence of and aren't needed for any other model, and in fact get in the way of our models of physics and quantum dynamics?


texts: How many GAIA Data Release 2 files are there?

texts:
I am using this index page to download GDR2 data files.

http://cdn.gea.esac.esa.int/Gaia/gdr2/gaia_source/csv/

But it keeps giving a different number of links. Sometimes it shows 16,000 files, sometimes 31,000 and the max I have seen is 61,237. What is the correct number I should download? GAIA GDR2 documentation does not list the total number of files available.

Interestingly, if the index lists less than 60k files, it still has the closing /html tag so it would appear I have a complete index file, but the contents are not complete.


texts: Is Mars brightest from Earth at opposition, perihelion or somewhere in between?

texts:
After reading this article, I was inspired to run some simulations at NASA's JPL Solar System Simulator to see Earth's and Mars' orbits from above.

From what I could visually determine (from the JPEG images), Mars is at opposition on July 26, but its perihelion is sometime around September.

When (and where in their orbits) is Mars brightest from Earth this year?


texts: The contents of potential "message" in the Wow! signal lost?

texts:
The Wow! signal is still an enigma.

I understand the incredibly high signal strength it entails. This during some time-period because the sweep of the alignment (or rather the receiving station). That could explain the rise and falloff. This because the horns tracking of that part of sky, once, because of earths movement.

The signal never returned. Everybody understands the narrow band of the signal, and the frequency of the hydrogen line. This incredible signal strength was logged, but only that, the spectacular strength.

But the contents of said signal was not logged, it was not even capable of recording that. Is this right? Is there a possibility that the burst could have carried information, however unlikely? It could have carried a one-time "message"? Is this correct?


texts: Helioseismology of the Sun, what is actually measured?

texts:
Wikipedia's article Helioseismology outlines several ways to analyze, data but is fairly thin on the nature of the measurements themselves.

Are there hundreds or thousands of high resolution spectroscopic measurements of the Doppler shift of some emission or absorption line distributed across the disk, or can this be done using low resolution spectroscopy to generate some variation in temperature or brightness?

I'm interested in a basic appreciation of the nature and granularity of the data (e.g. 0.1 angstrom in wavelength, 1 second of time, 1 minute of arc, etc.) and what kind of instruments collect said data.

The answer doesn't necessarily have to be long, just something to get me started.


texts: Do we know how old the matter that makes us is?

texts:
Last weekend, I had the pleasure of visiting an observatory where there was a series of lectures to cover up the fact that there was too much cloud cover to actually observe much at all.

One of these lectures involved the life-cycles of stars, which mainly involves Hydrogen and Helium.

Out of curiosity, I raised the question

  How old is the matter that make us and where does it come from?

The "where" is covered by Nucleosynthesis, but the "how old" was shrugged away with "maybe 2-3 sun lifetimes".

Is it possible to more accurately estimate the age of the matter that made up our solar system's accretion disk?  Or is the likely source of the supernova/collisions involved unlikely to be known?


texts:
Is it possible to establish one's longitude and latitude by observing the stars? Can you use observations of the stars to find the relative distance between two locations, which would be useful in map-making? What kind of crude tools would help (a sextant is too complex, but maybe a backstaff?)

What difference would there be if you were doing this on a planet that was not the Earth, and had a different location in the galaxy, a different day and year length?


texts: What is the bright object above the moon in Sudbury, Ontario?

texts: There is a bright object above the moon in Sudbury, highly visible to the eye, nothing else as bright in the sky except for the moon. I'm looking around 6 and 7am.  What is it ?


texts: Why is the shadow of the earth not visible during sunsets?

texts: I would imagine that during a sunset, for a brief moment of time, there would be a shadow of the earth seen in the clouds but I have never seen this. Why does it not happen?


texts: Why do some stars twinkle more than others?

texts:
This is a child's question I failed to answer. When observing some stars with naked eye, some stars (e.g. Regulus) appeared to blink significantly more than others, but I did not have the patience to identify them. Those stars twinkling more were on different positions on the sky, not next to each other. The sky was clear and cloudless, with quite a bit of ambient light of the neighbouring houses.
I tend to say that brighter stars seems to blink more, and the atmopheric effects are simply easier to perceive. But is that really correct?
Related

Why do stars appear to twinkle?

Why doesn't the moon twinkle?

Why do stars twinkle but planets don't

https://en.wikipedia.org/wiki/List_of_brightest_stars


texts: On gravitational wave radiation and arrangement of galaxies post- big bang

texts:
Orbits of planets and stars decay due to gravitational wave radiation. An elliptical orbit would become more circular with time. This is especially observed well in binary systems.

Taking the example of binaries, we can compute the rate of change of eccentricity of the orbit and the rate of change of the semi-major axis as a function of time. If we consider a highly elliptic orbit of a binary, with eccentricity, $\epsilon\rightarrow{1}$, we can prove that with time the orbit would eventually become a circular one, as follows:

  The power emitted by gravitational waves is given by:
  $$P_{GW}= \frac{c^5}{G}\left(\frac{GM}{c^5l}\right)^5$$
  Very compact binaries will lose energy rapidly by GW radiation.
  If we assume that the two bodies making up the binary lie in the $x-y$ plane and their orbits are circular ($\epsilon=0$), then only non-vanishing components of quadrupole tensors are:
  $$\boxed{Q_{xx}=-Q_{yy}=\frac{1}{2}(\mu)a^{2}\cos2\Omega t}$$ and
  $$\boxed{Q_{xy}=Q_{ya}=\frac{1}{2}(\mu)a^{2}\sin2\Omega t}$$
  Where $\Omega$ is the orbital velocity, $\mu=\dfrac{m_{1}m_{2}}{m}$ is the reduced mass and where $m=m_{1}+m_{2}$
  The luminosity of the system can be deduced as:
  $$L^{GM}=\frac{32}{5}\frac{G}{c^5}\mu^2 a^4\Omega^6=\frac{32}{5}\frac{G^4}{c^5}\frac{M^3 \mu^2}{a^5}$$
  The latter part is obtained from Kepler third law: $\Omega^2=\frac{GM}{a^3}$
  As the gravitating system loses energy by emitting radiation, the distance between the 2 bodies shrinks at a rate:
  $$\boxed{\frac{da}{dt}=\frac{64}{5}\frac{G^{3}M\mu}{c^5 a^3}}$$
  The binary would hence colase at a time :
  $$\tau=\frac{5}{256}\frac{c^5}{G^3}\frac{a_{0}^4}{\mu M^4}$$

  By applying a similar treatment to elliptical orbits , the following can be computed:
  $$\left<\frac{da}{dt}\right>=-\frac{64}{5}\frac{G^{3}m_{1}m_{2}(m_{1}+m_{2})}{c^{5}a^{4}(1-e^{2})^{\frac{7}{2}}}\left(1+\frac{73e^2}{24}+\frac{37e^4}{96}\right)$$

  $$\left<\frac{de}{dt}\right>=-\frac{304}{15}\frac{G^{3}m_{1}m_{2}(m_{1}+m_{2})}{c^{5}a^{4}(1-e^{2})^{\frac{5}{2}}}\left(1+\frac{121e^2}{304}\right)$$

  Solving this system of ODE's ultimately results in the equation:

  $$T(a_{0},e_{0})=\frac{12(c_{0}^4)}{19\gamma}\int_{0}^{e_0}{\frac{e^{29/19}[1+(121/304)e^2]^{1181/2299}}{(1-e^2)^{3/2}}}de\tag1$$
  Where $$\gamma=\frac{64G^3}{5c^5}m_{1}m_{2}(m_{1}+m_{2})$$
  Upon solving this one can find the time taken for the orbit to decay into a circle from an ellipse due to gravitational wave radiation.

A similar treatment can be done for the orbital decay of the planets of our solar system. Now my question is if we apply time reversal symmetry, the orbits become more and more elliptical and eventually tend to become a straight line(an elliptical orbit with $\epsilon\rightarrow{1}$) with a chunk of mass(moving in the straight line) from which all the planets and the Sun  of our solar system arose. Now the Milky Way "orbits" the Local Group, which in turn "orbits" the Virgo Supercluster.
Beyond that, the expansion of the universe starts to dominate over gravitation.

So reversing time to a space after the Big Bang, there must have been many galaxies and solar systems that were forming; so according to the above result the mass chunks from which galaxies, stars and planets arose must have had a highly elliptic orbits. Thus, the spread of matter in space must have been along approximate lines(which were orbits for the massive mass chunk a that formed them). This moves towards a linear arrangement of galaxies, contrary to what is observed. Can anyone please resolve my doubt. Any help is appreciated.


texts: What sources of turbulence exist within the Interstellar medium (ISM)? Which ones are physically the most important for newly forming stellar systems?


texts: Was the radiant of the Perseid meteor shower ever near Vega thousands of years ago?

texts:
The ancient Chinese story of the Weaver Girl (Altair) and the Cowherd (Vega) tell of starcrossed lovers who are only allowed to meet once a year on the 7th day of the 7th (Chinese) lunar month, roughly around early August, when magpies form a bridge across the silver river (the Milky Way.) This is celebrated in the Qixi festival in China (August 9th in 2016.)

Because of the timing, I'm suspicious that the magpies represent the Perseid meteor shower (August 12th in 2016.)  But I just got my first planisphere, and Altair and Vega are pretty far away from Perseus (although Perseus is still on the Milky Way.)  The Wiki article and others that I've looked into on Weaver Girl and Cowherd does not connect the magpies to any meteor shower.

But the Weaver Girl and Cowherd story is pretty old - up to 2600 years according to Wikipedia.  I don't think the Perseids have always been in Perseus due to the precession of the equinox, but 2600 years ago maybe the "Perseid" shower was closer to the southern triangle, or is that too far away to be explained only by precession of the equinox?

I'm very new to astronomy and I'm not Chinese, but I love this story and I'm looking for more information.

Thanks!


texts: Is Nomad data heliocentric or geocentric?

texts:
I'm wondering if the data in star catalogues are heliocentric equatorial coordinates or geocentric equatorial coordinates. Also given the distance between stars compared to the distance between the sun and the earth, would this be a big deal to mix up the two?

http://en.wikipedia.org/wiki/Celestial_coordinate_system


texts: What was the entity/particle/state at the time of the Big bang?

texts: At the time of the big bang if there was an original entity, what were it's attributes(if applicable) - mass, electric charge, color charge or spin? Can the current matter and dark energy together account for the original entity or was some of the original stuff lost?


texts: The Andromeda Galaxy is tens of thousands of light years in diameter. That suggests that the most distant stars in the Andromeda galaxy is up to tens of thousands of years "behind" in their orbit around the galaxy, compared to the closest stars. Have anybody generated pictures of the galaxy, which shows the approximate real position of stars? Is this effect visible when looking at the Andromeda galaxy through an amateur telescope?


texts: Is there a verification of the solar system barycenter as it is theoretically determined?

texts:
The barycenter of our solar system is not the sun, but a changeable location outside of the center point of the sun. A often used diagram of that relation is shown below, originated from the Wikipedia article.

Does exist one or more practical quite observations that confirm the accordance of the real changing of position of the barycenter of the solar system in relation to the center point of the sun with the theoretical results?


texts:
As discussions and answers to How large does refraction become in radioastronomy? point out, it is difficult to do radio astronomy much below 30 MHz (or 10 MHz depending on how aggressive you are in correcting for ionospheric effects like wavefront distortion and scintillation) from Earth's surface due to our ionosphere, which has made AM and short-wave radio listeners and ham radio operators happy for almost a century by being so reflective.

But that doesn't necessarily mean that there's no interesting radio astronomy way below 30 MHz.

Question(s):

What can be learned from low frequency radio astronomy available outside of Earth's ionosphere?
Are there currently any proposed or planned space-based low-frequency radio-telescope efforts?


texts: I am trying to make a software generated star chart, and want to include the outline of the milky way as shown in many such star charts.  Does anyone have any idea where to obtain the data for that?  The data must be available somewhere if star charts already have them.


texts: How long does lunar opposition surge last? Are there measurements of the full Moon getting suddenly brighter?

texts:
Wikipedia's opposition surge is a short article and forwards shadow hiding and coherent backscattering as proposed mechanisms, but it doesn't really explain how much the brightness of the Moon actually "surges" and how quickly it happens.
I was reminded of this by this interesting answer.
Question: Are there measurements and good quality plots of lunar opposition surge? How fast is it? How long does it last?

I don't think the coherent backscattering is the predominant mechanism for visible light; it's primarily seen in radar1. I'll be happy to be proven wrong, but the shadow-hiding seems to be pretty easy to embrace in photographs.
Images from https://space.stackexchange.com/q/20071/12102 and answers there. First is taken by the Yutu rover on the Moon (Chang'e 3 mission) second is a famous image (it's cropped, a reflection in a visor of the photographer, first lunar selfie!) and the third is from What causes Hyabusa-2's close-up images of Ryugu to be dark in the corners?
See also

Are there ANY verified satellite images of visible light coherent backscattering from Earth?
Now what do we think? Is this purple dot really caused by Cassegrain optics and coherent backscattering?
What's the story behind this Apollo-era image?
Lot 345 of Christie's “Voyage To Another World” auction supposedly has the only photo of Armstrong on the Moon, but isn't this one also? this was interesting...
1Clementine Bistatic Radar Experiment "coherent backscatter opposition effect"


texts: What is the composition of the Universe in terms of Types Of Stars?

texts:
There are

Main Sequence Stars
Red Giants
White Dwarfs
Red Dwarfs
Neutron Stars
and other types of Stars.

How many % of stars are Main Sequence? How many are Red Giants? and so on.

I searched around and found some info, but it's confusing me. For example,
I found that 90% stars are Main Sequence Stars, but at the same time, Red Dwarfs are the most common stars in the universe with 70%. What is true?


texts: Why it is not even theoretically possible to travel in past but possible to travel in future ?


texts:
What is the definition of the "solar mixing ratio"?

Specifically, I'd like to know why the ratio of H2 and He is 0.84 and 0.16 at the "solar mixing ratio" as shown in Table I in The Galileo Mission.


texts:
I'm considering a problem involving star S, planet A, and a comet B. The orbits of the planet and comet are eccentric, with eccentricities $e_A$ and $e_B$. I'm trying to calculate the average change in position that A should cause in B. I'm assuming $M_A \gg M_B$, so B should have negligible effect on A.

If I start using a coordinate system and assume S is at the origin, and A and B have semimajor axes $a_A$ and $a_B$ then A's orbit has polar equation $$r_A(\alpha) = \frac{a_A(1-e_A^2)}{1 - e_A\cos(\alpha)}$$

And if the angle between the major axes of A and B (the axes connecting their foci) at S is $\theta$ then B's orbit has polar equation $$r_B(\beta) = \frac{a_B(1-e_B^2)}{1 - e_B\cos(\beta-\theta)}$$

Thanks to Newton I know that the acceleration on B toward A is given by $$\text{acc}(\alpha, \beta) := \frac{GM_A (\vec{x}_A(\alpha) - \vec{x}_B(\beta))}{\mid\mid\vec{x}_A(\alpha) - \vec{x}_B(\beta)\mid\mid^3}$$ where $\vec{x}_P(\phi)$ is my shorthand for the position of object P, $(r_P(\phi), \phi)$.

My initial thought was the position change is given by $$\int_0^{2\pi}\int_0^{2\pi}\text{acc}(\alpha, \beta) \; \mathrm{d}\alpha  \; \mathrm{d}\beta$$ and so the average position change would be $$\frac{1}{4\pi^2}\int_0^{2\pi}\int_0^{2\pi}\text{acc}(\alpha, \beta) \; \mathrm{d}\alpha  \; \mathrm{d}\beta$$ however a colleague suggested that this assumes that the distribution of A and B in their orbits is uniform, which it isn't. So I did some calculations and worked out that object P has probability distribution $$\mu_{P}(\theta) = \frac{(1 - e_{P}^{2})^{3/2}}{2\pi (1-e\cos(\theta))^2}.$$

Armed with this knowledge I now think that the average effect of A on B is the change in position given by $$\frac{1}{4\pi^2}\int_0^{2\pi}\int_0^{2\pi}\text{acc}(\alpha, \beta)\mu_A(\alpha)\mu_B(\beta) \; \mathrm{d}\alpha  \; \mathrm{d}\beta.$$

That said I'm not sure if I'm missing anything or if my whole approach is off, and so I thought I'd ask here if my reasoning and ultimately my concluding integral seem correct. I'm a pure mathematician so I'm not very comfortable sorting out whether an application of theory has been properly applied to a real world situation. Any input is most welcome.


texts: Do gravitational waves have distinct bands or parameters from which a source redshift can be inferred?

texts:
In optical astronomy, much (most?) electromagnetic radiation is emitted at well defined frequencies, and this can be used to infer a redshift for the source, and hence its recessional velocity, age, or distance.

I haven't read anything specifically suggesting well defined source frequencies for gravitational wave astronomy, which would allow a redshift to be calculated the same way. I assume there would be a redshift effect since like EM radiation a gravitational wave propagates at (or exceedingly close to) the speed of light. But then again it propagates as a ripple in spacetime rather than a phenomenon embedded in spacetime, so perhaps not.

Do gravitational waves exhibit a frequency shifting effect akin to a redshirt, due to cosmic expansion, and which can be used to infer distance/time?
If so, do they also exhibit some kind of discreet originating frequencies, or some way that we can infer their originating frequencies, to compare the received signal and deduce the amount of redshifting they have undergone?


texts: Can we (theoretically) spin the black hole so strong that it will be broken apart by centrifugal force?

texts: I can't imagine the forces involved in black holes' lives. So please, help me to find out, if it is possible or not to destroy black hole in this specific way.


texts: What was up with the lights in the sky tonight?

texts: I and my boyfriend where looking up in the sky tonight around 8:30 am. We noticed about 17 or more things, that looked like stars. They was perfectly spaced, and just vanished they was heading east. It lasted about 5 to 10 minutes. What could it be?


texts: What is the ultimate fate of a cluster of galaxies?

texts:
We're fairly aware clusters of galaxies drift apart due to space expansion, which will drive them out of each other's cosmic event horizon eventually, leaving them separate, alone, each a single "body" within its local observable universe with no means to communicate or influence other clusters.

The clusters themselves are held together by gravitational binding that keeps them from being internally torn apart by space expansion, at least assuming density of dark energy doesn't increase drastically eventually.

What will happen to the cluster though? At first I assumed its black holes will swallow all its available matter, and connect into one single, lonely black hole. But then, neither are black holes ultimately eternal, due to Hawking Radiation, nor do they swallow all the stars, supposedly most stars escaping their gravity well.

Well then, what current theories say a local cluster of galaxies will transform into as t→∞ ?


texts: For focusing the image of a telescope, one can use a Bahtinov mask. How does this mask work, and how did Mr Bahtinov get the idea to cut such a peculiar shape? Is it possible for me to design my own focusing masks?


texts: How to convert from Black body intensity to MJy/sr?

texts:
How does one convert from  ${\rm ergs\cdot cm^{-2}\cdot s^{-1} \cdot sr^{-1}\cdot Hz^{-1}}$ to ${\rm MJy \cdot sr^{-1}}$
and from ${\rm ergs\cdot cm^{-2}\cdot s^{-1} \cdot sr^{-1}\cdot cm^{-1}}$ to ${\rm MJy \cdot sr^{-1}}$


texts: On Venus, what substances are likely to be found that are liquid in the temperatures and pressures and are there pools of the stuff?

texts:
It's clear that there is no water on Venus. But with temperatures hot enough to boil lead, well, couldn't there be pools of lead on the surface? Or pools of other things, like sulfur? Is there any evidence that these might in fact be present?

Wikipedia's Venus lists the average surface temperature of 464 C (737 K) and the pressure at about 92 bar (9.2 MPa).

For lead and sulfur at 1 atmosphere, the melting points are

            melting            boiling
sulfur   388 K  115 °C       718 K   445 °C
lead     601 K  327 °C      2022 K  1749 °C

In the search for life, it's assumed that liquid water is needed. Liquid, sure, but I wondered if it needed to be water. And I saw a video about silicon-based lifeforms. Unlikely, the conclusion was, because, as I recall, the p-orbitals are too small, so it can't form chains the way carbon does. But Venus (for one location) has higher temperatures and pressures than on Earth, and I wondered if that might squish things into reactions we don't see at STP.

All of that second paragraph isn't really part of the question, but that's what motivated me to ask.


texts: Why does Wikipedia state that the Solar System has one unconfirmed planet?

texts:
I was looking at exoplanets on Wikipedia, and found this surprising information:

The table states that the Sun has 8 planets (obviously) and one unconfirmed planet. Why is this here? I think it's Planet Nine, but I also have underlying thoughts that this planet is Pluto, a disputed dwarf/real planet. Is this correct?


texts: What unit is used for region Surface brightness in DS9?

texts: The value of the surface brightness is in what unit? I hope it is in Rayleighs. Is that true?


texts: Why are stars still distant from one another?

texts:
Why are stars so far from each other? Shouldn't gravity pull them closer over time? And if the effects of gravity are negligible is there an explanation why stars have to be so distant from one another?
The closest star is Alpha Centauri (I think) and it is 4.4 light years away.


texts: Did Einstein supported Big Bang Theory cosmological model?

texts:
Einstein made many predictions, including gravitational waves and the possibility of black holes.

Relativity is taken into consideration for the Big Bang model, so did Einstein agree with it or did he have a different view?


texts:
I know of three sets of stable orbits in a binary star system: orbiting closely around star A, orbiting closely around star B, or orbiting distantly around both stars (and their mutual center of gravity) at once.

Is there a fourth set of stable orbits, around the mutual center of gravity, but inside both stars' orbits?


texts:
In this article, the author writes:

  This galaxy is so distant that it would have been invisible without the gravitational lensing effects of a large galaxy cluster called Abell 1689, located nearly 2.2 billion light years away in the constellation Virgo. The cluster’s enormous mass acts as a natural telescope by bending and amplifying light from objects behind it, like A1689-zD1.

To me this doesn't quite sound the same as what a telescope is doing.

My question is: How can a galaxy in-between our view and the galaxy behind it have a 'lensing effect'?


texts: Have we ever observed a body, such as a large asteroid, "hitting" the Sun?

texts: Some other SE questions about launching ICBM's into the sun got me wondering whether we have ever observed an object on a path that intersected with the Sun? How close did it get?


texts: Are warm circumstellar debris disks still able to be detected at sub-millimeter wavelengths?

texts: At these wavelengths a survey is more sensitive to detecting relatively cool disks (T<100k), but would a telescope operating in the sub-millimeter be less sensitive to warm debris disk than say, a mid IR telescope?


texts: Do planets form around lone stars, multiple stars, or both? Do we know this yet?

texts:
Since about 1990 astronomers been able to detect planets around other stars, using a couple different techniques, which is amazing.

By this point, do we know whether planets form only around single systems, only around multiple systems, or both??

(Surprisingly you can't find this general issue explained in popular writing. I think it's "obvious background knowledge" to experts, hence just doesn't get outlined explicitly in popular writing.)

So, as of mid 2016:

(i) About how many exoplanets do we know of (say, with good certainty),

(ii) In fact, how many (if any) of these are around single-star systems?

(iii) How many (if any) are around multiple-star systems?

If indeed it is possible to have planets in multiple systems: is it just a planet going around one of the stars, or is "all the way around both" known?


texts: What wavelengths are used to search for distant solar system objects like KBOs and Oort cloud members?

texts: What wavelengths of light are used/most suited to search for distant solar system objects like KBOs and Oort cloud members? I suppose they are brightest in reflected sunlight, so they are best searched for in visible and near infrared. Is that correct?


texts: Is antimatter also attracted by gravitational field?

texts:
As we all know matter is attracted by the gravitational field of black hole. So, my question is: Is there any possibility of a black-hole attracting antimatter?

Can you explain how/why please?


texts: Are red clump stars horizontal branch stars?

texts:
The wikipedia article about horizontal branch stars says:

A related class of stars is the clump giants, those belonging to the
so-called red clump, which are the relatively younger (and hence more
massive) and usually more metal-rich population I counterparts to HB
stars (which belong to population II).

Whereas the article about the red clump says:

The red clump giants are cool horizontal branch stars

This image below also suggests that red clump stars are horizontal branch stars. Is this assessment corrent, and so is the first wikipedia article wrong to suggest that red clump stars are not themselves (but "counterparts to") HB stars?


texts: How can I find out which known stars lie within or close to M8

texts:
I'm looking for a way to find stars within a certain region of space, the Lagoona Nebula in this case, based on what we currently know. Are there some databases that allow you to make such queries?

Please note that I'm not looking for stars that can be seen as being near M8 from earth, but for a list of known stars that are thought to actually be near or part of M8 itself.


texts: Is it possible that a star system ejects an object out of the galaxy?

texts: I know it is possible that a star system ejects a planet out of the star system and the planets becomes rogue planets and orbit around the galaxy. Is it also possible that a planet gains enough speed because of ejecting and escapes out of the galaxy?


texts: Is it possible that some stars are already black holes yet we see light emitted from before becoming a black hole?

texts:
For stars we see that are burning fuel at a fast rate, that are very bright, (or any star for that matter sufficiently far away from us) could it be that they are already black holes (the sufficiently large ones) and we are just seeing the light emitted during it's main phase? So, if so, is it safe to say that most of the stars are already dead that we see?

Edit: I appreciate all the answers. This has been a fascinating read. I also hate having to select a "the answer" when I gleaned something from all of them.


texts: Why does Titan's atmosphere not start to burn?

texts: If Titan has a methane atmosphere and seas of methane, then why doesn't some meteoroid / chemical reaction ignite and blow the whole thing?


texts: What is the cut-off point of $\frac{\mathrm{d}F}{\mathrm{d}r}$ beyond which no tidal locking will occur?

texts:
I am a beginner, please pardon my non-use of proper astronomical terms. I would appreciate proper edits to my questions.

I have learned that when the $\frac{\mathrm{d}F}{\mathrm{d}r}$ is a very big negative number, that would mean that the far side will have much less gravitational tug than the near side of a satellite. The differential gravity or $\frac{\mathrm{d}F}{\mathrm{d}r}$ will be as follows

$$\frac{\mathrm{d}F}{\mathrm{d}r} = \frac{-2 GMm}{r^3}$$

If we compute $\frac{\mathrm{d}F}{\mathrm{d}r}$ of Earth-Moon system we get $-1.308 \times 10^{-5} \ \mathrm{M_o^2G/AU^3} $

I was surprised to know that Ganymede, being almost 3 times as far away from Jupiter as our Moon is from Earth, still is tidally locked.

If we compute $\frac{\mathrm{d}F}{\mathrm{d}r}$ of Jupiter-Ganymede system we get $-1.565 \times 10^{-6} \ \mathrm{M_o^2G/AU^3}$

I then wondered what is the $\frac{\mathrm{d}F}{\mathrm{d}r}$ of Sun-Earth that our Earth is not tidally locked to it and I found $-2.422 \times 10^{-8} \ \mathrm{M_o^2G/AU^3}$

Which is a very small negative number so the difference in tug is not significant.

So what is the minimum $\frac{\mathrm{d}F}{\mathrm{d}r}$ (or I guess "maximum") value or a boundary value at which the object can be tidally locked and beyond that, the tidal locking won't occur?


texts: What is meant by "Radial Direction" of a galaxy?

texts:
There is a scientific journal article having a line:

  "In order to study the star formation scenario in the radial direction
  of the LMC...."

What is meant by 'radial direction' of a galaxy ?


texts: When was the parallax of a star first measured?

texts:
Telescopes like Gaia measure the parallax of stars with a great precision. But for stars that are beyond 11 kpc, their parallax is still too small to be measured. With Earth-bound telescopes, only the nearest stars have a visible parallax. Was the parallax of any stars measured before the telescope era?
When was the parallax of a star measured for the first time?


texts: How to find period of a pulsar from a given time sampled voltage data?

texts:
I've a data file with 2 columns, each containing time sampled voltage output (a sequence of real numbers) from a radio telescope, so there is a pair of telescope which are measuring the signal here. The source of the data is a pulsar and I've to find its period and dispersion measure.

I'm an amateur trying to understand what is really happening in the method. The data is time sampled at the Nyquist sampling frequency (2B) and the signal is measured at central frequency f and broadband B.

I studied the cross correlation between the two voltages V1 and V2 from essential radio astronomy, but didn't quite understand the method. Do we have to evaluate the cross correlation function of each reading and then plot it with time to get the pulse profile? Or do we have to take fourier transform of each pair of voltage(how?) and then plot it to get the pulse profile? We find period of a pulsar from its average pulse profile, right?

It is also written that to find pulsar timing, we've to fold data from many pulses modulo instantaneous pulse period (how can we do that if we don't know the pulse period in the first place?). This sentence is everywhere and I'm not able to understand it each time.

I'm confused about the appropriate series of steps to be done here to get the pulsar period and dispersion measure. Could you give me some insight?

Thanks.


texts: Why do all the stars near this comet show up as four separate dots in a row?

texts:
After seeing this tweet about our not-so-new but at least newly-discovered "mini-moon" I scrolled down and saw the tweeted image below.

Since the object is the known short-period comet 114P/Wiseman–Skiff I'm guessing that the telescope guiding could have been pre-calculated and continuous, and yet the stars appear as four discrete and equally-spaced compact spots instead of continuous streaks.

Writing in the image says 2020 Jan 18.22 and 4x30s

What might be the explanation and reason for this?

Could these be stacked images resulting from a clever night of observing, perhaps bouncing back and forth between multiple objects so that each can be observed over an extended period of time?

Or do the four discrete spots per star allow for better astrometry than one long streak?

Or both? Or neither?

tweet

  Comet 114P/Wiseman-Skiff tonight at the Catalina Sky Survey 0.68m Schmidt.


texts: When are gravitational waves emitted during a merger of black holes?

texts: When black holes are merging, and emitting gravitational waves, are we seeing those waves start when the event horizons are merging, or when the singularities are merging?


texts: Angular momentum in planetary disk formation

texts:
Reading about the formation of planetary disks, one of the major problems, it seems like, is the evacuation of angular momentum. Aparently planets can't form with the amount o angular momentum the system has in its early stages. I think I understand where that excess comes from, the collapse of the nebula onto itself and provoking a spin. Then there are many hypothesis on how it's evacuated, which are mostly pretty logical.

Now my question, as a beginner in the study of physics, is this: Why does the angular momentum even need to be evacuated? If the angular momentum is too big, why can't planets still form? Does this have something to do with too much kinetic energy in the system?
Thank you!


texts: How to find the temperature of a planet accounting for the atmosphere?

texts:
Recently, I started writing a program to generate star systems, and I need a formula to find the approximate surface temperature of a planet. I know of several formulas for this, for example this one based on the formula for equilibrium temperature I found from a site called burro.case.edu:
$$T_{P}=(T_{\odot})(({1-a})^{1/4})\left( \sqrt{\frac{R_{\odot}}{2D}}\right)$$
Where $T_{\odot}$ denotes the parent star's temperature in Kelvin, $a$ denotes the dimensionless bond albedo, ${R_{\odot}}$ denotes the parent star's radius in AU (not kilometers/solar radii), $D$ denotes the semi-major axis in AU, and $T_{P}$ denotes the resulting computed planet temperate in Kelvin.
For example, if we plug  in the relevant numbers to Mars ($T_{\odot}=5778,a=0.25,R_{\odot}=0.0046547454,D=1.524$), we get an estimated temperature of Mars as 210.127 K, which is nearly spot-on with Mars's real temperature of 210 K. But if I plug in the formula for Venus, it gives a wildly off answer of about 260 K. In fact, this is briefly mentioned on the linked page:

Let's try this for Venus. Putting in the numbers (a=0.6, Tsun=5770K, Rsun=7x105 km, D=0.72 AU) we get the equilibrium temperature of Venus = 260 K.
The surface temperature of Venus = 740 K. Whoa! Where'd we mess up??
For that matter, the equilibrium temperature of the Earth is 255 K (or ~ -1 F). Something's wrong!

Obviously this happens becausing trying to estimate a true temperature from the equilibrium temperature results in ignorance of the atmospheric pressure. Venus's atmosphere is about 93 atm, resulting in a runaway greenhouse effect and heating the temperature. On a smaller scale, the same thing occurs for Earth as it has a pressure of 1 atm. The temperature calculation for Mars worked well because it has a pressure of only 0.01 atm making the difference minimal.
So I started searching on the web for other sources which have bias terms accounting for the atmosphere, but I still can't find anything. All I currently have  is a very rough approximation based on this Youtube video:
$T_{P} = 255(\frac{L^{1/4}}{\sqrt{D}})+50\sqrt{P}-50a$
Where $L$ denotes the star's luminosity with respect to the Sun, $P$ denotes the planet's atmospheric pressure with respect to Earth, and all the other variables are the same as they were in the last approximation. But this is still far from perfect, Venus's temperature has a ~15 K deviation from the real temperature. So, is there any more accurate formula for the atmospheric temperature of a planet?


texts: What theories are there in explaining the formation of the Pluto-Charon system?

texts:
After reading the question and answer to Effect of Charon on Pluto, a further question is this: how is the Pluto-Charon relationship explained?

Specifically, what theories are there in how did the Pluto-Charon system was formed?

Additionally, does this explain the presence of other moons (Nix, Hydra etc)?


texts: Why do  the Earth or any other planet continue to revolve around the sun or a star in general and that also with constant time period? What is the reason behind it? Other than gravitational force is there any other guiding force?


texts: How can a supernova cause a gravitational wave?

texts:
With the recent results of LIGO, I read a lot how limited the detection capabilities where and how one could only expect to measure the most extreme events like black hole mergers and supernovae.

I get the black hole mergers, since here we see an ever higher frequency gravitational interference of two large bodies. But how can a supernova cause a gravitational wave? All the mass is still there, it is just thrown out  isn't it? So from far away shouldn't the mass still be concentrated on a relatively small place and thus the gravity from the recently exploded star should remain unchanged, right?


texts: How to calculate position of an unknown star knowing positions of some other stars from an image?

texts:
Suppose I know the coordinates (right ascension and declination) of the stars marked with red. How should I calculate the coordinates of an unknown star, marked with yellow?

Searching over the web, I found a technique called least squares plate constants. I guess plate constants dates back before CCD cameras. Unfortunately there was only little explanation. However, if I understood the matter correctly, I should assume the stars are on a surface of a sphere imaged to a plane? Please help :)

Also, how should I calculate the distances between the stars?

The original image is from Wikipedia.


texts: What is the difference between equatorial and ecliptical coordinate system?

texts:
Of course, I know what the ecliptic is and what equator is. The problem is I still can not understand why we need two systems for measuring the same thing - are each of them useful only in specific occasions? For example, which is preferred in marine navigation? I am interested in this practical side of the question

Can you give an example which positions a star, using both coordinate systems?


texts: How do we know that a natural satellite cannot be a hollow object?

texts: In their 1966 book, Intelligent Life in the Universe, Carl Sagan and Iosif Shklovsky state that "a natural satellite cannot be a hollow object." How do we know this is absolutely true?


texts: Why dust is optically thin in Far Infrared wavelengths?

texts: What is the actual meaning of the statement 'Dust is optically thin in the Far Infrared (FIR) over most of the Galaxy'? Kindly Help


texts: Are there any planets or objects orbiting in Goldilocks zone?

texts: I was wondering whether there were any objects that orbit the "goldilocks zone" besides our home planet earth, and how the goldilocks zone of our solar system is defined.


texts: North Pole Right Ascension/Declination to axial tilt conversion

texts: Wikipedia , on several asteroid articles, claims that right ascension and declination of the North Pole can give you the axial tilt of the object. What formula is used to convert these 2 values into the axial tilt? (Ceres, for example, has an R.A. Of 291 degrees and a declination of 59 degrees; its axial tilt is approximately 3 degrees. How is this figured?


texts: Which is the closest exoplanet to Earth?

texts: The closest star to Earth (after the Sun) is well-known: Alpha Centauri, 4.3 light years away. But what about the closest exoplanet to Earth, outside of the Solar System?


texts: Given the horizontal coordinates of two objects in the sky, how does one go about calculating the angular separation between them?


texts: Is night sky getting brighter day by day?

texts:
Night sky is dark because light from distant stars has yet not reached our earth. Light emitted by stars formed after the instant of creation i.e. 13.7 billion light years ago is on the way to reach us. As light from these distant stars reach us, night sky must keep getting brighter.
How true is this? And to what extent? Can one day night sky become as bright as a day?


texts: Are horizontal branch stars fully convective?

texts:
I know that RR Lyrae stars are convective (this is suspected to be part of the cause of the Blazhko effect), but I would like to know more generally:

Are RR Lyrae fully convective?
Are all HB stars convective, and fully convective?

The detailed articles I've found so far to try to answer this question are all quite focused on RR Lyrae variables and so don't readily provide the answers to these questions as far as I've seen.


texts: Who first observed another planet rotating?

texts:
We knew the earth rotates, but the moon is tidally locked.

To tell that another planet is rotating, you have to be able to distinguish its surface (I guess?). Who first did this?


texts: Is broadcasting the location of Earth to potential extraterrestial civilization regulated?

texts:
How many times have we broadcast the location of Earth to the outer space? Are there any regulation in doing so?

I know so far we have send the Arecibo Message, and any receiver can trace the message sender's direction by the direction with which the message is received. But have we ever pin point our exact location of the galaxy and send it as a message?


texts:
I'm a grad student and I feel like this is an extremely elementary question that I should know, but I admit that I don't. Occasionally in science I find myself solving an equation for some answer where it feels like the units don't add up.
For instance, in a spatially flat matter-dominated universe with nonrelativistic matter, the accepted age of such a universe is:
$$t_0 = \frac{2}{3}H_0$$
where $H_0$ has units of ${\rm km} \cdot {\rm s}^{-1} \cdot {\rm Mpc}^{-1}$.
Why am I getting a time from this? I know it works, I know this is correct. This isn't the only example of this I've ever seen, but it is the freshest in my memory. It conceptually makes sense, but usually I'm in the habit of doing meticulous dimensional analysis and just wondered if there's some deep metaphysics or philosophy behind when that applies and when it doesn't.


texts:
I know a repeat of the 1859 Carrington Event (coronal mass ejection) would pose a severe risk to power grids and telecommunication networks but wouldn't pose an immediate threat to life.
If a large solar flare can release 10e32 ergs of energy, is there an energy level where a solar flare or CME would pose an immediate threat to any life on the Earth that doesn't live below sea level.


texts: Can the Hubble Sphere ever extend over the cosmic event horizon?

texts:
I am struggling to reconcile the Hubble volume with the idea of a cosmic event horizon.
As I understand, the Hubble volume is increasing over time because Hubble's constant is decreasing. This should allow photons emitted in superluminal regions to enter into the Hubble volume and become subluminal.
The cosmic event horizon, as I understand it, is the point where any light emitted beyond the cosmic event horizon will never reach Earth.
So, can a galaxy emit light that can reach us, but then pass over the cosmic event horizon, such that any light it emits after this point will never reach us? And, if the Hubble volume is increasing in diameter, why can't it overtake the cosmic event horizon, allowing the light in this region to now travel subluminally?


texts: Air molecules per cubic foot at a certain altitude above the surface

texts:
The Earth's and other celestial bodies' atmospheres become thinner the higher you go. Imagine you were able to see atoms/molecules. At what altitude above the Earth's and Mars' surface would you see few enough molecules per cubic foot so you'd be able to count them? If you were, say, 500 miles (805 km) above the Earth's surface, how many air molecules per cubic foot (30 cm³) would you see?
Or, one can ask vice versa: at what altitude above the Earth would you see, say, a 100 air molecules per ft³? Which gas doesn't matter, I'm asking for all atmosphere molecules within the certain volume. And since even interplanetary space isn't perfect vacuum, what would it look like there, e.g. halfway between the Earth and Mars orbit?


texts: How much clearer are stars in earths orbit?

texts: As a child I remember my parents taking camping in California, and upon stepping out of the car I was awestruck at how thick the Milky Way galaxy was and at the number of stars everywhere. I've always wondered how much clearer it could get. What percentage of visible light from stars are absorbed by all of earths atmosphere at the clearest locations on earth? Are we taking about a small percentage or perhaps a significant amount?


texts: The Sun or Our Galaxy revolves around what?

texts: Like every planet they are in orbit of something or another planet like our solar system every planet revolves around the sun, my question is: what's the name of the planet or object which the sun revolves around; if the sun is stationary or if our galaxy is in motion, around what it is revolving.


texts: Point Spread Function size: Full Width Half Maximum (FWHM) vs Sigma

texts:
I having been studying some astronomy papers related to galaxy observations and I realized that every time they want to express the size of the Point Spread Function (PSF) of a system which can be approximated by Gaussian they use the Full Width Half Maximum (FWHM) and not the sigma. Is there a reason behind that?

Thank you.


texts: Is there some substance on Venus or some kind of material, because neither bodies have any source of light.


texts:
My title doesn't do my question justice in explaining, but the kind of satellite system I'm thinking of is two rings on two separate axis, much like (if you've seen it) the film 'Treasure Planet' shown below.

My guess is no, as the ring of a planet matches it's axis, but I was wondering whether or not some other phenomenon or influence could cause this formation.


texts: Is gravitational wave frequency always equal to double the orbital frequency?

texts: If the binary does not evolve into merger stage (i.e.: it is still a steady binary), does the gravitational wave frequency have to be 2*orbital frequency? Could the frequency be, for instance, 2/3*orbital frequency and steady, rather than a chirp? And if not, could the frequency 2/3*orbital frequency be the result of interactions with other waves of this binary? The frequency range is about several milihertz, low frequency.


texts:
I'm searching for long- and non-periodic sungrazing comets, like the Kirch comet of 1680 and the (meanwhile destroyed) Comet ISON. If you know a(ny) long-/non-periodic sungrazing comet(s), let me know here, please.
I'd prefer to learn about sungrazers that are not members of the Kreutz group, but you can also post Kreutz comets, perhaps that would be good too. Such an example would be Comet Lovejoy.
Also welcome are posts on sungrazing comets that once were long-periodic but have changed to periodic due to another planet's gravity influence.
Thank you.


texts: Looking back in time by looking further away

texts:
I have been pondering the question of how we can look back to the state of the early universe for some time. So I came here and found, to my delight, that several such questions have been asked, such as this one. Fortunately the answers to that (and other questions) has helped immeasurably, but I have one lingering area of confusion:

I often hear, in cosmological TV documentaries (such as Horizon in the UK) words such as "The further away we look, the further back in time we are looking. Our technology has now advanced to the point we can see objects so far away they began emitting their light just [n] after the big bang" [this is not a quote from a programme but my synthesis of what I have heard].

My question is this: I understand the limitation of c and light taking finite time to reach us from distant objects. Having now understood that inflation caused the early universe to "move" so far away we are only now receiving its light; How can today's technology evolve into tomorrow's technology and enable us to see "older" light as we can only see what is arriving at the Earth right now. I.e. if a time-traveller turned up today bearing a better telescope from the future, I should not be able to use it to see further away and therefore back in time, because it is dependent on the age of the light arriving at my location.

The only solution I can think of is that due to tricksy inflation rate effects, the light of successively further objects is arriving at, essentially, the same time (when looked at over a universal age). But that doesn't seem to make sense and even if it did how would we sort the older light from the newer light from the same object if it is all arriving at once (notwithstanding red shift, which I do (hopefully) understand)...


texts: How to figure out what something in the sky is?

texts:
A while ago I saw something in the sky early in the evening. At first, I thought it was a plane since it seemed far too bright to be a star - no other stars could be seen. However, it didn't move, so I ruled out that possibility.

It seemed to be relatively close to the horizon so I considered if it was a very high pole with some sort of light at the top, but after I walked for a few kilometres it was still at the same location.

During the few hours I looked at it, it moved a bit across the sky.
I considered a satellite, but it didn't seem to move quite enough of what I anticipated a satellite would move.

Since then, I have seen it every evening when I remember to look for it.

Apart from this object only the moon could be seen. To me, it seemed like the moon moved in the same arched way across the sky as the object, but I could be wrong on the.

I have seen several questions on this site where someone asks "what was it?" which has been closed with "too broad". So I am not going to ask what it was.
Instead I am curious about what I can do to figure out what it is? Which observations is relevant to make when looking at such objects? As someone who is curious about the sky, but does not have any instruments for doing any actual measurements can I actually do anything?

The first picture shows the object in the sky.

The second picture shows the same object, but where I have indicated the arch the object moved in as well as the moon and how that moved.


texts: When was the variation of apparent diameter of the moon first measured?

texts:
The orbit of the Moon around the Earth isn't quite a circle. Since distance between the Earth and the Moon changes over time, the apparent diameter of the Moon also varies very slightly. With a digital camera, you can observe this by counting pixels. However, this variation is very small and not easily noticed.
Who discovered that the angular diameter of the Moon varies over time?


texts: The martian blueberries (only blue in false color) found by the Opportunity rover in Meridium Planis are formed by slow evaporation in mineral-rich liquid water. They obviously formed early in the Martian history, but do we know more precisely when these blueberries formed?


texts: How does one go about submitting a name for a newly-discovered lunar crater?

texts: Suppose I discover an uncatalogued crater on the lunar surface, and decide to name it. How would I submit my proposed name to the IAU?


texts: Which direction do the constellation revolve in?

texts:
If I am standing looking towards the East from the Northern Hemisphere, do the constellations appear to rotate towards the West via the North or the West via the South or am I completely off target all together?

Why do we say Capricorn is in December at the Solstice? Where do we see it in the sky in the Northern Hemisphere in December and likewise would we see Aquarius in that same position in February etc..?


texts: How often do they move most/all of ALMA's dishes?

texts:
The Phys.org article Unknown treasure trove of planets found hiding in dust reports on recently published radio measurements (~1.3mm) of planetary systems forming around stars in Taurus:

Scientists base this scenario of how our solar system came to be on observations of protoplanetary disks around other stars that are young enough to currently be in the process of birthing planets. Using the Atacama Large Millimeter Array, or ALMA, comprising 45 radio antennas in Chile's Atacama Desert, the team performed a survey of young stars in the Taurus star-forming region, a vast cloud of gas and dust located a modest 450 light-years from Earth. When the researchers imaged 32 stars surrounded by protoplanetary disks, they found that 12 of them—40 percent—have rings and gaps, structures that according to the team's measurements and calculations can be best explained by the presence of nascent planets.

The article ends by saying:

Going forward, the research group plans to move ALMA's antennas farther apart, which should increase the array's resolution to around five astronomical units (one AU equals the average distance between the Earth and the sun), and to make the antennas sensitive to other frequencies that are sensitive to other types of dust.
"Our results are an exciting step in understanding this key phase of planet formation," Long said, "and by making these adjustments, we are hoping to better understand the origins of the rings and gaps."

Changing from 15AU to 5 AU suggests a roughly factor of 3 increase in the overall scale of the pattern of antennas. The linked paper in ApJ Feng Long et al. has a preprint in ArXiv as well, which says:

All observations were obtained from late August to early September 2017 using 45-47 12-m antennas on baselines of 21∼3697 m (15∼2780 kλ), with slight differences in each group (see Table 1).

Several years ago I remember reading about the design of ALMA and there are a lot more places to put dishes than there are dishes. I think there were five optimized configurations that all looked to have a similar spiral shape, but each one was maybe a factor of 2 or 3 larger than the previous.
Since the dishes take time to move and setup and calibrate, this probably doesn't happen that often.
Question: How often does it happen? How often do they move most of ALMA's dishes to change resolution?
If there's a site that shows how ALMA is set up now, and what the next setup will be and when, that would be great to know about as well.

above: "Until recently, protoplanetary disks were believed to be smooth, like pancake-like objects. The results from this study show that some disks are more like doughnuts with holes, but even more often appear as a series of rings. The rings are likely carved by planets that are otherwise invisible to us." Source Credit: Feng Long/ALMA


texts: What can a lunar-based telescope do? If the band is ultraviolet and it can do high time sampling exposure, maybe it can be used to get distance for a star with stable pulses. However, the star should be bright and its distance should not be large.


texts:
Scientists strongly suspect that several moons in our solar system have frozen-over oceans of water-ammonia mixture. I've also read speculations on the possibility of surface water-ammonia oceans on exoplanets and exomoons. What I haven't seen a lot of is a discussion on how those oceans might behave differently than the water oceans of Earth.

More specifically, how would the surface water-ammonia ocean behavior differ from our water ocean behavior in terms of:

Ice(s) formation
Evaporation
Surface and underwater currents
Varying ratios of water-ammonia based on temperature or other
factors

I'd like to know if this has been studied and if these behaviors have been described in papers, books, or in-depth articles on this topic. Especially anyting that goes into the specifics of the water/ammonia ratios.

I'll add some personal speculation here. While it is not part of the primary question stated above, it helps to explain the type of problem I'm trying to understand, so may be helpful to narrow down which resources may be most useful.

When subject to increasing levels of heat would the ammonia molecules
in the ocean evaporate before the water molecules, causing warmer
areas of the ocean to have an increased amount of H2O relative to
NH3? (And the cooler areas like the poles having an increasing ratio
of NH3 to H2O).
If the above was true, might the equatorial waters that have less NH3
facilitate the formation of water-ice? If so I could imagine
exoplanets having water-ice-covered equators because they were
warmer, and the cooler poles might be ice-free thanks to the greater
amount of NH3 in the ocean.
Would the ice formed on the ocean be composed of a molecular mix
matching that of the ocean (with H2O and NH3 in the same ratio as the
ocean), or would some molecules freeze-out first, possibly causing
different layers or areas of varying types of ices?
Might the poles of some water-ammonia planets have "continents" of
water-ammonia ices that are frozen from ocean bed up to the surface
(thanks to the fact that frozen ammonia is more dense than liquid,
allowing for entire sections of ocean to freeze solid)?

If my speculation is correct (and I could be wildly wrong since I don't yet understand how a mix of water-ammonia actually behaves) then an exoplanet with an equatorial band of water-ice, mid-latitudes of liquid ocean, and solid polar-caps/continents of water/ammonia ices could be possible. The ammonia in the ocean would evaporate more quickly near the warmer equator, causing that area of a global ocean to have a greater ratio of H2O, and the ammonia precipitation and ocean might freeze at the poles, causing the build up of vast polar continents of ammonia/water ices. I would imagine the currents would function very differently than on earth, with surface and deep-level currents of varying ammonia-water ratios and temperatures. I I also see the possibility of frozen ocean beds in the more ammonia-heavy parts of an ocean, with the surface unfrozen.

Of course the ratio of ammonia to water will be very important. I'd love to see specific information on that!

Any informed clarification appreciated!